The present invention relates to a lubricating construction for outboard motors. More specifically, the present invention relates to a lubricating construction for outboard motors which supplies a sufficient amount of a lubricating oil to valve end sections of the outboard motor.
A four-cycle engine includes a valve gear device provided in a cylinder head. Respective sliding portions in the valve gear device are continuously subjected to lubrication with a lubricating oil conducted by way of oil passages.
In conventional outboard engines, so-called valve end sections, that is, contact points between heads of intake and exhaust valves and ends of rocker arms for opening and closing the valves, in a valve gear device lack means for supplying a lubricating oil directly thereto. Lubrication of the valve end sections depends upon the splashing of a lubricating oil which scatters in the cylinder head.
A cam shaft is vertically positioned in a conventional engine, such as an engine for outboard motors, in which a crank shaft is positioned vertically. A thrust force acting in an axial direction of the cam shaft is born by a thrust bearing construction provided at a lowermost portion of a cylinder head, in which the cam shaft is housed. The thrust bearing construction supports a lower end of the cam shaft.
However, lubrication of valve end sections cannot be adequately performed only by the splashing of a lubricating oil which scatters in a cylinder head. Specifically, with a vertical-type engine, such as an engine mounted on an outboard motor, upwardly positioned valve end sections for cylinders cannot be adequately supplied with a lubricating oil. As a result, such valve end sections must be have a greater hardness relative to the normal case. This increases manufacturing costs and prevents use of common parts.
Where a thrust bearing construction is arranged at a lowermost portion of a cylinder head, it is difficult to mount and dismount a cam shaft. This results in an unfavorable assembly and maintenance quality.
Also, in the case where a thrust bearing construction is arranged above a cylinder head, a sufficient amount of lubricating oil does not spread over sliding surfaces of the thrust bearing construction and the cam shaft for the above described reasons. Therefore, such conventional arrangements are unfavorable due to the possibility that abrasion may result on poorly lubricated sliding surfaces.